Numerous processes are already known for the preparation of arylamine/formaldehyde condensates, and in particular aniline/formaldehyde condensates, by reaction in the presence of aqueous acid catalysts followed by processing of the condensation mixture by distillation after neutralization of the catalyst and removal of the organic phase. Polyamines based on diphenylmethane which are obtainable by these processes are used primarily as starting materials for preparation of the corresponding polyisocyanates. Among the polyisocyanate mixtures based on diphenylmethane series used in polyurethane chemistry, those which contain a high proportion of 4,4'-diisocyanatodiphenylmethane and a low proportion of o-isomers such as 2,2'- or 2,4'-diisocyanatodiphenylmethane are particularly valuable starting materials. The diamine content of the polyamine mixtures obtained by arylamine/formaldehyde condensation can be controlled by suitable choice of the ratio of arylamine to formaldehyde put into the process. The o-isomer content of the polyamine mixtures particularly the amount of 2,2'- or 2,4'-isomers in these mixtures, is primarily a function of the strength and concentration of the acid catalyst used. A high degree of protonation (degree of protonation = percentage of total quantity of nitrogen atoms present as ammonium groups) generally leads to an increased proportion of p-isomers in the end product of the process. One disadvantage of a high degree of protonation, however, is that it inevitably requires a greater effort for neutralization of the reaction mixture. The disadvantage of preparing polyamine mixtures containing a high proportion of diamines by using a large excess of arylamine as starting material is the increased distillation required to remove unreacted amine.